It is of advantage for the transmission of information to use optical carrier waves extending from the infra-red to the ultra-violet radiations and with which it is possible to obtain bandwidths considerably greater than those obtained with conventional electronic systems. It is known that optical modulators can be produced by utilising electrical or magnetic effects. Connections from one point to another are also currently being established by means of optical fibres. Finally, the detection of optical radiation is also possible.
By contrast, there are at present few arrangements for directing the exchanges of optical radiation between emission and detection.
For this purpose, an advantageous basic arrangement is an arrangement performing the function of an optical circulator which is a non-reciprocal arrangement, that is to say an arrangement of which the ways of transmission are not the same according to the direction and the polarization of the incident radiations.
It is already known that a birefringent element may transmit luminous rays in different directions depending upon the polarization of the rays. For example, a uniaxial material deflects the so-called "extraordinary" rays but does not deflect the so-called "ordinary" rays. For turning the polarization of rays, there are reciprocal elements which are made of a naturally birefringent material, such as half-wave plates, quarter-wave plates, etc. Reciprocal elements transmit the rays in both ways of a given direction and, by a good choice of the orientation of the axis and the direction of propagation, can turn the polarization of an incident ray through a constant angle in opposite ways for opposite ways of transmission. At the contrary, there are non-reciprocal element made in materials in which a birefringence is produced by an exterior electromagnetic field. For instance, by creating a magnetic field around certain magnetic materials, one can obtain a Faraday-effect rotator which turns the polarization of an incident ray through an angle of which the value depends upon the propagation length in the rotator and the way does not depend upon the way of transmission in the rotator. Intrinsically and produced birefringent plates can be made in a bulk or a thin layer technology.